The use of acoustic (e.g., audible and/or ultrasonic) measurement systems in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications, is well known. Such acoustic measurement systems are utilized in a variety of downhole applications including, for example, borehole caliper measurements, measurement of drilling fluid properties, and the determination of various physical properties of a formation. In one application, acoustic pulses may be generated at one or more transmitters deployed in the borehole. The acoustic responses may then be received at an array of longitudinally spaced apart receivers deployed in the borehole. Acoustic logging in this manner provides an important set of borehole data and is commonly used in both LWD and wireline applications to determine compressional and shear wave velocities (also referred to as slowness) of a formation.
In the analysis of acoustic logging measurements, the received acoustic waveforms are typically coherence processed to obtain a time-slowness plot. In a time-slowness plot, also referred to as a slowness-time-coherence (STC) plot or a semblance plot, a set of several signals from the array of acoustic receivers is processed with the incorporation of separate time shifts for each received signal. The separate time shifts are based on a slowness value assumed for the purpose of processing the waveforms. The processing provides a result, known as coherence, which can signify the presence of a discernable signal received by the separate receivers. In this manner compressional and shear wave arrivals can be discerned in the received waveforms, leading to determinations of their velocities. The determined compressional and shear wave velocities are related to compressive and shear strengths of the surrounding formation, and thus provide useful information about the formation.
Like any other downhole measurement, acoustic logging data are subject to various sources of noise both internal and external to an acoustic logging tool. Internal noises may include instrument noises and interference of nearby downhole tools. External noises may include drilling noises, mud flow noises, noises from non-steady tool positions or varying mud properties, and local variations in near borehole rock properties. These noise sources are commonly at least somewhat random in nature and can significantly erode the signal to noise ratio of a detected acoustic signal.
Therefore, there exists a need for an improved method for making acoustic logging while drilling measurements in a subterranean formation. In particular, there is a need for an improved method for making acoustic logging while drilling measurements that can achieve improved signal to noise ratios during logging while drilling operations.